The present invention relates generally to a semiconductor device, and more particularly to a field effect transistor consisting essentially of a Group III-V compound semiconductor.
Conventionally, a high electron mobility transistor (HEMT) is known as a field effect transistor (FET). The HEMT has, on a semi-insulating GaAs substrate, a hetero junction between a non-doped InGaAs channel forming layer and an electron supplying layer consisting essentially of a semiconductor which has a lower electron affinity than that of InGaAs and is doped with an n-type impurity to a high concentration. The HEMT has a high operation speed and a low noise ratio by using as a carrier a two-dimensional electron gas (2DEG) having a high electron mobility formed in the InGaAS channel forming layer of a high purity. In general, AlGaAs, InGaP or InGaAlP is used as a material of the electron supplying layer.
FIG. 2 is a schematic cross-sectional view showing a conventional HEMT having an electron supplying layer made of InGaP. The HEMT is produced in the following process.
First, a non-doped GaAs buffer layer 202, a non-doped InGaAs channel forming layer 203, a non-doped InGaP spacer layer 204, an Si-doped n-type InGaP electron supplying layer 205, a non-doped InGaP Schottky contact layer 206, and an Si-doped n-type GaAs ohmic contact layer 208 are formed in sequence on a semi-insulating GaAs substrate 201 by metal organic chemical vapor deposition (MOCVD).
Then, a resist pattern (not shown) is formed on the GaAs ohmic contact layer 208. A source electrode 209 and a drain electrode 210 are formed through an electrode metal deposition step, a lift-off step and an alloying step.
Further, a resist pattern (not shown) is formed by electron beam exposure so that only a part of the n-type GaAs ohmic contact layer 208 is exposed. Using the resist pattern as a mask, the n-type GaAs ohmic contact layer 208 is removed by etching (recess etching), so that the surface of the non-doped InGaP Schottky contact layer 206 is exposed. A gate electrode 211 is formed on the exposed surface of the Schottky contact layer 206. In general, Ti is used as a material of the gate electrode 211, in consideration of the barrier height between the gate electrode and the InGaP Schottky contact layer 206, leak current or stability.
However, although Ti has a comparatively high melting point, thermal diffusion into InGaP is not avoidable. According to reliability tests, the threshold voltage was varied. It has become clear that these results of the tests are due to the non-uniform diffusion of Ti into an InGaP layer. The electrode can be formed of any metals other than Ti, which have the Schottky characteristic, for example, Mo having a high melting point or Pt. The present inventors manufactured a transistor by way of trial with these metals, but could not find a metal superior to Ti in terms of the initial characteristic and reliability characteristic, since all these metals have problems in barrier height, leak current or stability.
Diffusion of metals other than Ti also causes a considerable problem in the device characteristics like the aforementioned problems of Ti.
As described above, in the conventional HEMT, Ti is generally used in consideration of the barrier height relative to a Schottky contact layer, leak current or stability. However, thermal diffusion of such a metal element into a semiconductor layer is not avoidable. It is known, through reliability tests, etc., that the use of such a metal has a problem of deterioration of the device characteristics (for example, the threshold voltage is varied) and a decrease in planar uniformity.